In a feedback control system, a controller obtains the value of a controlled variable, compares that value with a setpoint, and adjusts the value of a manipulated variable in order to drive the controlled variable toward the setpoint. In the context of a process control system, adjustment of the manipulated variable generally involves adjusting a valve. For example, if the controlled variable is the level of fluid in a tank having an intake valve and an outlet valve, the manipulated variable can be the volume rate of flow into or out of the tank. Both of these variables are ultimately manipulated by adjusting the position of a valve. A valve for controlling the flow of fluid is thus a critical component in the control of a processing plant.
To control a valve, the controller sends a signal to a positioner, which is a mechanical device intimately associated with the valve that moves in response to the signal. When the positioner moves, it changes the position of the valve and hence, the value of the manipulated variable controlled by that valve. This change in the manipulated variable results in a corresponding change in the controlled variable. The controller then measures the value of the controlled variable and, if necessary, sends another signal to the positioner to correct the value of the manipulated variable. This process of measurement, followed by correction on the basis of the measurement, is at the heart of a feedback control system.
Unfortunately, it is possible for the controller to send the positioner a signal and for the positioner to do nothing, to move an incorrect amount, or, in the worst case, to move in the wrong direction. The failure of a positioner can, of course, be detected by measuring the value of the controlled variable and observing whether that value is inconsistent with the expected value of the manipulated variable. However, in many processes, there may be significant lag time or dead time. In such processes, it may be some time before the controller realizes that the controlled variable is not changing as expected. During this lag time, significant damage may occur. For example, if the valve controls the flow of coolant in a nuclear power plant, by the time the temperature of the coolant rises, the core temperature may already be dangerously high.
It is therefore desirable to detect the failure of a positioner as soon as possible. Because the positioner is typically hidden from view, this is most readily accomplished by having the positioner transmit a signal verifying that it has, indeed, moved to the location specified by the controller. This generally requires a signal transmitter mechanically coupled to the positioner such that when the positioner is in the desired position, an electrical signal is transmitted to the controller, to an alarm panel, or to some other appropriate location. In a typical signal transmitter of this type, a protruding signal flag coupled to the positioner moves into engagement with an electromagnetic switch when the positioner reaches a desired position.
A disadvantage of known signal transmitters is the difficulty encountered in adjusting the location of the signal flag to accommodate variations in valve positioners. In known signal transmitters, adjustment of the flag location generally requires access to the top and sides of the signal transmitter. In addition, when the flags are loosened for adjustment, they move relatively freely and are therefore difficult to adjust independently of each other with precision.
Because of the difficulty in adjusting the signal flags with precision, the mechanical motion of the positioner needs to be amplified so that small errors in positioning the flags do not result in large errors in the perceived position of the valve. This, in turn, requires that a system of gears having a gear ratio selected to amplify the mechanical motion of the positioner be interposed between the positioner itself and the signal transmitter. This gear system provides yet another source of possible failure, adds to the cost of the signal transmitter, and, because the mechanical resolution of the system is limited by the spacing between the gear teeth, decreases the overall resolution of the signal transmitter.
An additional disadvantage of known signal transmitters is that the signal flags are mounted in a manner susceptible to vibration. Exposure to such vibrations can eventually cause the signal flags to become misaligned. As a result, such signal transmitters require frequent maintenance.
A position indicating apparatus according to the preamble of claim 1 is known from GB 2 265 204 A.
It is thus an object of the invention to provide a signal transmitter in which the signal flags can be adjusted independently of each other with sufficient precision to eliminate the need for an amplifying gear between the positioner and the signal transmitter.
It is a further object of the invention to significantly reduce the sensitivity of the signal flags to vibrations.